Question: In chemistry class, the girl to boy ratio is $3$ to $10$. If there are $9$ girls, how many boys are there?
Solution: A ratio of $3$ girls to $10$ boys means that there is a group of $3$ girls for every group of $10$ boys. If there are $9$ girls, then there are $3$ groups of $3$ girls each. Because we know the ratio is $3:10$ , there must be $3$ groups of $10$ boys each. There is a total of $30$ boys in chemistry class.